The present invention relates to shaft seals, and more particularly to on-line liquid inject valves and the shaft seals provided in these valves.
Those skilled in the on-line process analysis art are well familiar with these valves, in which a sampling shaft having a channel defined transvesely therein reciprocates between a sampling position wherein a liquid stream flows across the shaft and through the channel, and an inject position wherein the channel traverses a length of a seal and delivers a liquid sample carried in the channel to, e.g., a gas chromatograph for analysis. In the best commercially-available valves, the seals around the sampling shaft have a fairly limited useful life (e.g., 50 or 60 thousand strokes in moderate service) before elements of the liquid stream to be analyzed begin to leak into the gas chromatograph because of the frictional wear between the sampling shaft and the seal.
In efforts to maintain an effective seal around the sampling shaft in these commercially-available valves as the seal is worn, several manufacturers provide a spring or springs to compress the seal and to force the seal into better sealing engagement with the reciprocating sampling shaft. As the seal is worn in these devices, however, the springs relax and exert less force on the seal, which creates an increasing susceptibility of the valve assembly to leaks into the gas chromatograph. An additional disadvantage of this type of construction is that the amount of force which can be exerted by a spring on a seal is limited. Still another disadvantage is that the springs provide a minimal level of flexibility in terms of adjusting the pressure to be placed on a given seal, and changing the pressure to be placed on the seal requires taking the valve apart.